Written by a parent, not a doctor. Nothing here is medical advice.

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Methotrexate — IV (Capizzi / high-dose)

Updated September 2026.

Also known as: HD-MTX, Capizzi, escalating methotrexate

The big infusions with the hospital stay. Blocks a vitamin cells need to make DNA; leucovorin rescues the healthy ones after.

Methotrexate through the line is the same folate blocker as the weekly pill, at a far higher dose. Folate is what a cell uses to make fresh DNA building blocks; methotrexate is shaped like folate and jams the enzyme (DHFR) that keeps folate usable, so dividing cells run out of parts and stop.

High-dose methotrexate floods the body, reaches places small doses cannot, including the fluid around the brain and spine, and gets into every dividing cell. It is followed by leucovorin, which rescues healthy cells, plus a lot of fluid and repeated blood tests to make sure the drug is clearing. The hospital stay is so the team can watch it clear. Capizzi methotrexate is a different style: a moderate dose stepped up from one dose to the next, without rescue, usually paired with asparaginase.

I made an everyday analogy and a more scientific visualization of IV methotrexate.

Everyday analogy

Methotrexate is like a pencil jammed in a shared sharpener.

Methotrexate is like a pencil jammed in a shared sharpener. Two seated, yellow-skinned workers copy house drawings at a shared workbench. They read their original sheets, make copies and sharpen their ordinary pencils in turn; three successful sharpening visits establish the routine. The worker on the right puts an ordinary pencil into a pot, lifts the teal pencil and tries the same sharpener. Its tip stays lodged in the feed opening. The worker returns the remaining teal pencil to the pot and takes back the ordinary pencil. Both workers keep copying until their points wear down. Each tries the blocked sharpener separately, then returns to the paper without a renewed point. The final faint marks leave both copies unfinished. A close-up makes the blunt point visible. Teal identifies methotrexate. Copying represents copying DNA; the sharpener represents an enzyme that keeps folate usable, and sharp pencils represent usable folate needed to help make DNA building blocks. The jam is a simplified picture of inhibition, not a permanently broken enzyme. The shared sharpener Read the original. Make a copy. Keep the pencil sharp. COPYING ROOM ORIGINAL COPY COPY ORIGINAL JAMMED Teal pencil = methotrexate Worn-down point No clear line to copy with UNFINISHED UNFINISHED The workers read, copy and sharpen their pencils again and again. A teal pencil jams the sharpener, like methotrexate blocking a needed step. Their points wear down. Sharpening fails, and the copies are left unfinished.

1 / 3 Copy and sharpen

The workers read, copy and sharpen their pencils again and again.

  • The teal pencil blocking the sharpener represents methotrexate. The jam shows a blocked process, not a permanently broken enzyme.

  • Copying a sheet represents copying DNA, the instructions a cell copies before it divides.

  • The sharpener represents an enzyme that keeps folate usable. Sharp pencils represent usable folate, which helps cells make DNA building blocks.

  • Without that help, DNA copying can slow or stop. This is a simplified picture of one action of methotrexate.

Press play below for a more accurate visualization.

Interactive visual

How high-dose methotrexate floods the body, jams the folate enzyme in leukemia cells, and why a rescue medicine follows

Poster for How high-dose methotrexate floods the body, jams the folate enzyme in leukemia cells, and why a rescue medicine follows
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## 0–10 seconds: A high concentration arrives High-dose methotrexate arrives from a blood vessel, so both leukemia cells and healthy cells in bone marrow encounter a large amount. ## 10–28 seconds: Two routes in; longer retention Methotrexate enters through folate carriers. At high concentrations, additional entry can occur without a carrier. Inside the cell, enzymes add glutamate tails to methotrexate. These tails help keep the medicine inside. ## 28–44 seconds: The normal folate pathway Normally, DHFR recycles folate into a usable form. The usable folate supports production of DNA building blocks. DNA-copying machinery uses those building blocks to extend a new strand. Watch a pale block settle into place. ## 44–66 seconds: The pocket jams; copying stops Methotrexate occupies the folate enzyme’s pocket, so folate cannot use it. The supply needed for DNA building blocks falls. As building blocks run short, the DNA-copying machinery slows and can stop. ## 66–82 seconds: Cell stress and timed rescue Severe DNA-copying stress can trigger self-destruction: mitochondria release signals, so caspases activate and dismantle the leukemia cell. Later, timed leucovorin rescue bypasses the blocked enzyme, so healthy cells can replenish usable folate and recover. ## 82–94 seconds: Sealed packets; quiet cleanup The dying leukemia cell seals itself into small packets, so cleanup cells can swallow them. The nearby healthy cell remains intact.

Side effects

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